EVIDENCE OF INTERNUCLEOSOMAL DNA FRAGMENTATION AND IDENTIFICATION OF DYING CELLS IN X-RAY-INDUCED CELL-DEATH IN THE DEVELOPING BRAIN

Newborn Sprague-Dawley rats received a single dose of 2 Gy X-rays and were killed 6 hr later. Dying cells were characterized by extreme chro matin condensation and nuclear fragmentation. Dying cells were distrib uted in the primary and secondary germinal zones and in other brain re gions. Among these latter, dying cells occurred in the cortical layers of the olfactory bulb, layers II-III and VIb of the neocortex, pirifo rm and entorhinal cortex, stratum oriens and pyramidale of the hippoca mpus, striatum, thalamus, amygdala, brainstem, internal granular layer of the cerebellum, and cerebral and cerebellar white matter. Dying ce lls were immature cells, neurons and glial cells (including radial gli a). In;situ labeling of nuclear DNA fragmentation identified individua l cells bearing fragmented DNA. Since the number of cells stained with this method was larger than the number of dying cells, as revealed wi th current histological techniques, it is suggested that nuclear DNA f ragmentation precedes chromatin condensation and nuclear fragmentation in X-ray-induced apoptosis. Furthermore, agarose gel electrophoresis of extracted DNA from irradiated brains showed a ''ladder'' pattern wh ich is typical of internucleosomal DNA fragmentation and endonuclease activation.